Do not know if your Mac is hooked up to the Niagara. I had the same issue on a secondary system with a Panamax e5100 surge protector. I disconnected the amp from the Panamax and the popping stopped.
Short?
I have a McIntosh integrated hybrid tube amp / VPI table / Revel speakers. When the AC comes on or off in the room i hear a loud pop. I used to think it was a scratch on the album but now I realize its some sort of elelctrical issue. I thought it might be a table grounding ussue but now it also happens occaisionally now when I'm not playing an album. So does the amp need to be grounded? And to what? Thanks.